Reflecting Galvanometer, Ballistic

Leeds & Northrup Co.

Philadelphia

Humboldt S-T-C; c.1930

Usage/History

The reflecting galvanometer is a sensitive
instrument for measuring current. It consists of a magnet coil
with an attached mirror suspended between the poles of a magnet
by a thin gold ribbon above and a coiled gold wire below. When
an electric current is introduced into the magnet coil it induces
a magnetic field, causing the coil to rotate in proportion to
the current. The mirror allows the user to see very small deflections,
and thus to measure very small currents, by observing a distant
reflected scale (one meter or more away) through a telescope.

Some contemporary/early descriptions of the Galvanometer
and its use are provided below:

The ballistic galvanometer is specialized in having a wide
coil characterized by a long period. It is designed to measure
the quantity of electricity (charge) discharged through the instrument
over a short period of time. Applications include the measurement
of capacitance, inductance by comparison with a standard, and
magnetic field intensities using a search coil.

Description

L&N Type P, Ballistic, student grade
moving coil reflecting galvanometer, serial number 730967. The
meter consists of a 7 1/8 by 4 5/8 by 7/8" magnet covered
by a bakelite plate with a center 1 3/4 by 3 3/8" window
and with a hollow suspension post of nickel plated brass (4 1/4
by 3/8") on top. The L&N logo is in raised letters against
a black polished region on the diamond stippled background of
the bakelite cover. There are raised polished borders around
the edge and the window. The mirror is 5/8" dia. There is
a white backing inside the meter with the following characteristics
noted (data hand entered) SENS. .0018uc/mm , C.D.R.X. 12000'~,
PERIOD 27.2 Sec, RESIST 2146~, CAT. NO. 2239D. The meter is mounted
on 6 1/4 by 9 by 7/8" bakelite backboard with two nickel
plated brass hangers at the top, two black painted telescope
arm brackets on the sides, two bakelite binding posts at the
bottom on either side of a leveling screw, a manufacturers label
on the bottom edge and a brass Humboldt S-T-C tag (4925) on the
lower left hand corner.

The meter is mounted on a Galvanometer base, Tripod, L&N
2126, made of cast iron, identical, with the exception of the
crinkle finish, to item 82315, pg. 742 of the 1960 (J-300) Cenco
catalog. An essentially identical base had a decal labeled: State
of California/Humboldt College/11xx1.

The meter was damaged in the Dec. 1994 Humboldt county earthquake:
the brackets were bent, and the upper suspension tube was snapped
off at the base where it screws into the meter frame. The brackets
were subsequently straightened. However the tube is held in place
by friction, and has not been repaired further.